Utility-scale solar power plants are predominantly configured as fixed-tilt ground mounted arrays or single-axis trackers. Fixed-tilt arrays are arranged in East-West oriented rows of panels tilted South at an angle dictated by the latitude of the array site the further away from the equator, the steeper the tilt angle. By contrast, single-axis trackers are installed in North-South rows with the solar panels attached to a rotating axis called a torque tube that move the panels from an East-facing orientation to a West-facing orientation throughout the course of each day, following the sun's progression through the sky. For purposes of this disclosure, both fixed-tilt and single-axis trackers are referred to collectively as axial solar arrays.
Excluding land acquisitions costs, overall project costs for utility-scale arrays include site preparation (surveying, road building, leveling, grid and water connections etc.), foundations, tracker or fixed-tilt hardware, solar panels, inverters and electrical connections (conduit, wiring, trenching, grid interface, etc.). Many of these costs have come down over the past few years due to ongoing innovation and economies of scale, however, one area that has been largely ignored is foundations. Foundations provide a uniform structural interface that couples the system to the ground.
When installing a conventional single-axis tracker, after the site has been prepped, plumb monopiles are driven into the ground at regular intervals dictated by the tracker manufacturer and/or the site plan; the tracker system components are subsequently attached to the head of those piles. Most often, the piles have an H-shaped profile, but they may also be C-shaped or even box-shaped. In conventional, large-scale single-axis tracker arrays, the procurement and construction of the foundations may represent up to 5-10 percent of the total system cost. Despite this relatively small share, any savings in steel and labor associated with foundations will amount to a significant amount of money over a large portfolio of solar projects. Also, tracker development deals are often locked-in a year or more before the installation costs are actually incurred, so any post-deal foundation savings that can be realized will be on top of the profits already factored into calculations that supported the construction of the project.
One reason monopiles have dominated the market for single-axis tracker foundations is their simplicity. It is relatively easy to drive monopiles into the ground along a straight line with existing technology. Even though their design is inherently wasteful, their relatively low cost and predictable performance makes them an obvious choice over more expensive alternatives. The physics of a monopile mandates that it be oversized because single structural members are not good at resisting bending forces. When used to support a single-axis tracker, the largest forces on the foundation are not from the weight of the components, but rather the combined lateral force of wind striking the solar panels attached to the array. This lateral force gets translated into the monopile foundation as a bending moment. The magnitude of the moment is much greater than the static loading attributable to the weight of the panels and tracker components. Therefore, when used to support single-axis trackers, monopile foundations must be oversized and driven deeply into the ground to stand up to lateral loads.
There are alternatives to monopiles available in the marketplace but thus far they have not been cost competitive. For example, in very difficult soils where costly refusals dominate, some solar installers will use ground screws instead of H-piles. As the name implies, a ground screw is essentially a scaled-up version of a wood screw or self-taping metal screw, with an elongated, hollow shaft and a tapered end terminating in a blade or point. The screw also has a large, external thread form extending from the tip, up the taper and even partially up the shaft to enable it to engage with soil when screwed into the ground. Such a prior art ground screw is shown, for example, in FIG. 1A. Ground screws like the ground screw 10 in 1A are manufactured and sold by Krinner, GmbH of Strasskirchen, Germany, among others. When installers encounter rocky soils or must install over bedrock, they predrill holes at the location of each ground screw and then drive the screws into the pre-drilled holes, attaching above-ground foundation hardware to the head of each screw.
When used in foundations for single-axis trackers, grounds screws like that in FIG. 1A are typically installed in adjacent pairs. The pairs are joined above-ground with an upside-down T bracket that presents a monopile interface for the single-axis tracker. This is seen, for example, in system 20 in FIG. 1B. Ft. Meyers, Fla. based TERRASMART installs foundations like system 20 using Krinner ground screws. While this may mitigate the problem of refusals, it does not optimize material savings and will only pencil out where less expensive alternatives won't work. Vertical foundations that support single-axis trackers must resist bending, whether made from H-piles or ground screws. Referring to FIG. 2B, when wind strikes the array, it generates a lateral force labeled FL in the figure. The magnitude of this force is equal to FL multiplied by the height of the pile above the point where the foundation is pinned to the ground (e.g., does not move). This force puts plumb foundation components into bending. Because structural members are generally poor at resisting bending, they must be overbuilt to withstand it.
Another proposed alternative to percussion driven H-piles and vertical ground screws, uses a pair of ground screws driven at acute angles to each other in an A-frame configuration. Unlike plumb monopiles or the double-screw foundation of FIG. 1B, an A-frame has the advantage of converting lateral loads into axial forces of tension and compression in the legs. This is seen, for example, in published U.S. Patent Application, 2018/0051915 (herein after, “the '915 application”). FIG. 1C shows the system described in the '915 application. In theory, such as system enables the legs to be thinner than those used, for example, in the system of 1B, because the legs are not subjected to bending. FIG. 2C is a force diagram showing how lateral loads are translated in an A-frame such as that in 1C. Lateral load FL puts tension on the windward leg and compression on the leeward leg. System 30 is potentially an improvement over plumb piles and parallel ground screws, however, any system that uses standard ground screws is at a costs disadvantage relative to other structures. Moreover, the ground screw's closed geometry mandates a separate pre-drilling step where direct driving is not possible. Therefore, in their current form, and with conventional rotary driving and drilling equipment, it is not possible for ground screws to achieve cost parity with monopile foundations in anywhere other than in the worst soil conditions, and even in those conditions, there is room for significant improvement.
In recognition of these and other problems, it is an object of various embodiments of this disclosure to provide a truss or A-frame foundation for single-axis trackers and other applications that realizes the benefits of ground screws in a less costly, more robust, and flexible form factor, as well as machines and methods for installing such foundations.